The oxygen evolution reaction (OER) is central to storing electrical energy via chemical bonds in energy carriers and fuels. Metal organic frameworks (MOFs) have drawn considerable attention as hybrid organic-inorganic catalysts that can mimic structure of biological oxygen evolving complexes, which are still over an order of magnitude more active than state-of-the-art OER electrocatalysts. Recently, metal-hydroxide organic frameworks (MHOFs) a new class of MOFs which combine layered hydroxides with aromatic carboxylate linkers that stabilize the structure via π-π interactions, have been shown to display three times the tunability of oxyhydroxide analogues, offering extensive opportunities for numerous applications. However, the long-term stability of these materials during OER is still unclear, which is paramount to understand for further rational design of MOF and MOF-derived catalysts.
In this study, we investigated Ni-based MHOFs with linkers of varying π-π interaction strengths to understand how these differences affect morphological, structural, and oxidation state changes of the materials during OER in basic media. We observed that all MHOFs undergo activation during electrochemical cycling, leading to two orders of magnitude increase in OER activity. Further utilizing advanced microscopic and both ex and in situ spectroscopic techniques, a strong correlation is found between the dissolution of linkers from the MHOF structure with the number of electrochemically accessible Ni sites and OER activity. We identify a transformation from the MHOF phase to a NiOOH_2-x-like phase during OER in which mono-µ-oxo motifs present only in MHOFs convert to the di-µ-oxo motifs of a NiOOH_2-x-like phase and conclude the OER active phase to be NiOOH_2-x. From our observations, we propose a new mechanistic scheme for the loss of MHOF-specific mono-µ-oxo motifs and phase transformation to from MHOF to NiOOH_2-x. The transformation is observed to be rapid and complete in MHOFs with weaker π-π interaction strengths, while MHOFs with stronger π-π interaction strengths undergo slower transformation and only at the near-surface regions. This work highlights the importance of linker selection in MOF design and provides valuable insight for understanding the driving force and mechanism of the decomposition of MOFs under electrochemical conditions, which can directly be applied for the future design of highly active, yet stable, MOFs for electrochemical applications and beyond.

Hongbin Xu

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